1. Field of the Invention
The present invention relates to a switching power source for generating a desired voltage by switching of DC voltage, voltage conversion, and synchronous rectification, more particularly relates to technique for protection of the synchronous rectification circuit.
2. Description of the Related Art
FIGS. 1A and 1B show the circuit configurations of conventional synchronous rectification circuits in switching power sources as disclosed in Japanese Patent Publication (A) No. 2003-189608 (secondary side circuits of transformers).
The circuit shown in FIG. 1A is a current doublers type synchronous rectification circuit. This circuit is the most generally used synchronous rectification circuit directly driving switch elements SW100 and SW200 (NMOS transistors) by an output of a secondary winding of the transformer. In the circuit shown in FIG. 1A, however, when the power source output is stopped, the charge stored in an output capacitor C0 passes through the secondary winding and turns on the switch elements SW100 and SW200, so sometimes induces breakage of these elements.
FIG. 1B shows a synchronous rectification circuit for controlling the ON and OFF states of the switch elements SW100 and SW200 not by directly driving the switch elements SW100 and SW200 (NMOS transistors) from the output of the secondary winding of the transformer, but by combining for example a not shown auxiliary winding and control circuit. In this synchronous rectification system, a rectification efficiency can be raised in comparison with the circuit shown in FIG. 1A.
However, in the synchronous rectification circuit shown in FIG. 1B as well, sometimes the drain-source voltages of the switch elements SW100 and SW200 exceed a withstand voltage level and induce breakage of these elements when no protection circuit is provided. Below, the mechanism by which breakage of the switch elements is induced will be explained with reference to FIGS. 2A and 2B.
FIGS. 2A and 2B are diagrams for explaining a current path in the conventional synchronous rectification circuit shown in FIG. 1B, in which FIG. 2A shows a case of no load output, and FIG. 2B shows a case after the transformer output is suspended. In the synchronous rectification circuit shown in FIG. 1B, when the power source supply has no load, since there is no current flowing in a load, the current flowing in a transformer Tm circulates through coils L10 and L20, switch elements SW100 and SW200, and the output capacitor C0 as shown in FIG. 2A. The currents flowing in the coils L10 and L20 become 0 when totaled in the reverse directions. At this time, assume that the switch element SW100 is in an ON state, and the switch element SW200 is in an OFF state.
At this time, when the drive signal on a primary side of the transformer Tm is suspended due to an abnormality, the output voltage of the transformer Tm immediately becomes 0V, but according to any configuration of the control circuit, the switch element SW100 sometimes was continuously kept ON since the time before that. Then, as shown in FIG. 2B, the current flows in reverse through the output capacitor C0, that is, the current of the output capacitor C0 is discharged through the switch element SW100. In such case, the switch element SW100 may break due to the rise of the discharge current unless the switch element SW100 is forcibly turned off. However, if the switch element SW100 is forcibly turned off here, there will no longer be a discharge loop for releasing the energy of the coils L10 and L20 (since the escape route of the current is lost), so there is a possibility that high voltages will be induced between the drains and sources of the switch elements SW100 and SW200 and these elements will be broken.